.\" Copyright (C) 2021 Stefan Roesch <shr@fb.com>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_readv 3 "November 15, 2021" "liburing-2.1" "liburing Manual"
.SH NAME
io_uring_prep_readv \- prepare vector I/O read request
.SH SYNOPSIS
.nf
.B #include <sys/uio.h>
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_readv(struct io_uring_sqe *" sqe ","
.BI "                         int " fd ","
.BI "                         const struct iovec *" iovecs ","
.BI "                         unsigned " nr_vecs ","
.BI "                         __u64 " offset ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_readv (3)
prepares a vectored IO read request. The submission queue entry
.I sqe
is setup to use the file descriptor
.I fd
to start reading
.I nr_vecs
into the
.I iovecs
array at the specified
.IR offset .

On files that support seeking, if the offset is set to
.BR -1 ,
the read operation commences at the file offset, and the file offset is
incremented by the number of bytes read. See
.BR read (2)
for more details. Note that for an async API, reading and updating the
current file offset may result in unpredictable behavior, unless access
to the file is serialized. It is not encouraged to use this feature, if it's
possible to provide the desired IO offset from the application or library.

On files that are not capable of seeking, the offset must be 0 or -1.

After the read has been prepared it can be submitted with one of the submit
functions.

.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I res
field will contain the result of the operation. See the related man page for
details on possible values. Note that where synchronous system calls will return
.B -1
on failure and set
.I errno
to the actual error value, io_uring never uses
.IR errno .
Instead it returns the negated
.I errno
directly in the CQE
.I res
field.
.SH NOTES
Unless an application explicitly needs to pass in more than one iovec, it
is more efficient to use
.BR io_uring_prep_read (3)
rather than this function, as no state has to be maintained for a
non-vectored IO request.
As with any request that passes in data in a struct, that data must remain
valid until the request has been successfully submitted. It need not remain
valid until completion. Once a request has been submitted, the in-kernel
state is stable. Very early kernels (5.4 and earlier) required state to be
stable until the completion occurred. Applications can test for this
behavior by inspecting the
.B IORING_FEAT_SUBMIT_STABLE
flag passed back from
.BR io_uring_queue_init_params (3).
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_prep_read (3),
.BR io_uring_prep_readv2 (3),
.BR io_uring_submit (3)
